net: dsa: mv88e6xxx: move VTU VID accessors

Add helpers to access the VTU VID register in the global1_vtu.c file.

At the same time, move mv88e6xxx_g1_vtu_vid_write at the beginning of
_mv88e6xxx_vtu_loadpurge, which adds no functional changes but makes
future patches simpler.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>